This invention relates to a direct parallel connection circuit of self-turn-off semiconductor devices or elements such as gate-turn-off thyristors (hereinafter referred to as GTO) and transistors, and more particularly to a direct parallel connection circuit of self-turn-off semiconductor elements wherein self-turn-off semiconductor elements designed to have different current capacities, respectively are directly connected in parallel, and turn-on and turn-off operations are adapted to be simultaneously performed to dispose any current capacity.
Direct parallel connection of GTO's having the same current capacity is previously known, and one example thereof has been proposed in U.S. Pat. No. 4,612,561.
FIG. 9a shows an equivalent circuit of the construction disclosed in the above patent. In FIG. 9a, GTO1 and GTO2 having the same current capacity are connected in parallel with their gates being connected to each other via a auxiliary gate wire Ga and their cathodes being connected to each other via a auxiliary cathode wire Ka. A turn-on power supply 3 and a turn-off power supply 4 are connected in series with each other. A transistor 5, a resistor 6, a reactor 7 and a thyristor 8 are connected in series between the power supplies. The midpoint between the resistor 6 and the reactor 7 is connected with the common gate terminal G of GTO's 1 and 2, and the midpoint of the power supplies 3 and 4 is connected with the auxiliary cathode wire Ka. A and K are main terminals to which the anodes and cathodes of GTO's 1 and 2 are connected, respectively.
The feature of this prior art resides in the provision of the auxiliary gate wire Ga and the auxiliary cathode wire Ka.
When a signal is supplied to the transistor 5, a gate turn-on current flows to the gate G whereby GTO's 1 and 2 having the same current capacity are turned on. On the other hand, when a signal is supplied to the thyristor 8, a gate turn-off current flows to the gate G whereby GTO's 1 and 2 are turned off. In this case, if GTO's 1 and 2 have substantially the same static/transient property, the currents i.sub.A1 and i.sub.A2 flowing through GTO's 1 and 2 are balanced, when GTO's 1 and 2 are turned on/off, by the action of the auxiliary gate wire Ga and the auxiliary cathode wire Ka, as shown in FIG. 9b. Therefore, the composite current capacity can be taken up to the value of the current capacity of each GTO multiplied by the number of GTO's connected in parallel (Actually, the above composite current capacity can be only near the possible maximum current capacity due to the derating considering the variations of the respective element property).
In FIG. 9b, v.sub.AK is an anode-cathode voltage of GTO's 1 and 2, i.sub.GON1, i.sub.GON2 are turn-on currents of GTO's 1 and 2, respectively and i.sub.GOFF1, i.sub.GOFF2 are turnoff currents of GTO's 1 and 2, respectively.
In the above prior art parallel connection system of GTO's, in order to preferably turn on/off the parallel connected GTO's, the current capacities of the respective GTO's must be the same. Therefore, if the on/off operation of 450 A is intended when there are two GTO's of 200 A current capacity and two GTO's of 300 A current capacity, the prior art system is not useful. Namely, using two GTO's of 200 A current capacity doesn't permit 450 A to be turned on/off. On the other hand, using two GTO's of 300 A current capacity permits the turn on/off but gives rise to redundant allowance, thereby causing disadvantage in cost.
All the same, it takes a long time to newly design, develop and fabricate GTO's of 225 A or 450 A current capacity. This cannot be a present solution.
The same problem will be also occur when transistors of the same current capacity are connected in parallel.